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PRODUCT DESIGN RECOMMENDATIONS
Resistance of metal edge systems to tear-off when secured to properly installed substrates is very important for wind resistance. The attachment of the wood nailer to the substrate should be sufficient to resist 100 pounds per foot. At outside building corners, nailer securement should be reinforced to resist at least 200 pounds per foot.
Although all roof edge systems should be tested according to the tests outlined herein, the following rules will generally produce roof edge designs with sufficient properties:
- Fascias, copings, etc. should be secured with continuous hook strips of 24 ga. steel, .050” aluminum or metal of equivalent strength at the bottom of the face edge.
- Hook strips or cleats should be secured with annular threaded nails long enough to penetrate the wood nailer at least 1 inch. Nail heads should be at least 3/16 inch in diameter.
- Hook strips may be secured with minimum No. 8 screws, long enough to penetrate the nailer 3/4 inch or penetrate metal 3/8 inch.
- Where velocity pressures are less than 45 lbs/sq.ft., hook strip fasteners should be placed no greater than 24 inches apart.
- Where velocity pressures are greater than 45 lbs/sq.ft. they should be spaced every 16 inches or closer. Note: There are some manufacturers whose designs are tested to achieve positive hold-down at 12 inches o.c.
- Where velocity pressures exceed 45 psf, add a screw through the back section (near the center) and at the center of each joint cover or splice plate.
- Fastener frequency should be doubled in corner regions.
- Nail heads should be larger than the narrowest dimension of the slotted holes.
- Fascia and coping sections should be spaced to allow for expansion around the screw, and from piece to piece.
- To ensure adequate holding, edge designs should include a drip edge which securely engages with the hook strip.
Wood nailers should be pressure treated lumber, minimum thickness 1 1/2 inch, and of sufficient width to extend at least 1/2 inch beyond the edge of the rear flange of the metal edge system used to secure the roofing membrane.
Nailers should be secured with corrosion resistant anchor bolts, countersunk into the wood nailer and attached with nuts and washers. Anchor bolts should be a minimum 1/2 inch diameter, and spaced no farther than 4 feet apart. If the wood nailer is wider than 6 inches, anchor bolts should be staggered to avoid splitting of the wood.
Nailers should be secured either to masonry or to steel decking according to the following requirements:
A. Masonry
When embedded into masonry, anchor bolts should be bent 90 degrees at the base or have heads designed to prevent rotation and slipping out. When hollow block masonry is used at the roof line, cores and voids in the top coarse should be filled with full density concrete. When embedded into light aggregate block, anchor bolts should be embedded a minimum of 12 inches into concrete fill.
When heavy aggregate blocks are used, bolts should be embedded a minimum of 8 inches.
B. Concrete and Gypsum Decks
Nailers should not be fastened to light weight concrete or gypsum decks. Instead, nailers should be anchored directly to wall structural members using fasteners whose size and locations meet the guidelines stated in Masonry “A” above.
C. Steel Deck
When nailers are anchored directly to steel decks, a steel angle should be installed to take the load. Attach the nailer to it with corrosion resistant anchor bolts that are securely attached to the steel angle. This angle should be specified according to the requirements of the local building codes.
When the deck is a minimum 22 ga. steel, the angle should be secured to the deck with fasteners having a minimum 360 pound pull-out rating. For lighter gauge decks, the fasteners should be tested for a minimum 360 pound pull-out with the type of deck specified. Fasteners should be installed on maximum 12 inch centers. Minimum 5/8 inch galvanized steel washers should be used under screw heads.
D. Additional Wood Members
Additional wood members, such as fascias, cant strips and stacked nailers, should be fastened with corrosion resistant screws having a pull-out resistance of at least 360 pounds per fastener. Screws should be staggered a maximum of 12 inches on center. Spacing should be a maximum of 6 inches on center in corner regions.
E. Nailerless Systems
When the metal edge system is attached directly to masonry or steel without the use of a nailer, its attachment should be tested to resist wind up-lift.
References
- Minimum Design Loads for Buildings and Other Structures, ANSI/ASCE 7-93
American Society of Civil Engineers, New York, 1994
- SPRI Fastener Corrosion Guidelines, 1988
MaintenanceThe roofing membrane, its continuity, and the roof’s attachment to the perimeter should be inspected periodically and repaired as needed. Roof debris which tends to collect along the roof perimeter should be removed to allow easy water runoff. Appliance attachments which may penetrate the water seal, induce electrolysis, or otherwise compromise the effectiveness of the roof edge system, should also be inspected and eliminated wherever possible.
DESIGN OPTIONS
The holding power of the metal edge detail is divided into two considerations:
- The resistance of the edge to outward and upward forces which tend to blow or peel the edge system off the substrate.
- The ability of the edge to resist the pull of the roof membrane inward towards the field of the roof.
High performance roof edge details should be selected from manufacturers who certify that the products they produce meet or exceed certain performance characteristics (based upon testing) of the building’s design requirements. Other designs may be used, provided they are tested and certified to meet the wind resistance and termination tests suggested in this document.
Calculation of Design Pressure
Design Pressure, P, is obtained by correcting the Theoretical Velocity Pressure, q, for a particular location on the building by use of a Pressure Coefficient, Cp.
P = Cp x q
Velocity Pressure is the theoretical pressure imparted by the kinetic energy of the wind. In practice, aerodynamics will cause actual wind pressures to differ from theoretical values at certain locations on the building. A building with a flat, level (or slightly sloped) roof will experience greater forces at the corners and eaves than on interior roof surfaces because of “eddy effects” at the eaves.
Velocity Pressure:
The theoretical Velocity Pressure, q, for a particular building is obtained from Table 3 (on next page) utilizing the Building Exposure category previously discussed and determined under Design Considerations, Section E “BuildingLocation”. Because of channeling between buildings and its possible effect upon the uplift at the roof edge, it is recommended that theoretical velocity pressures of Exposure “C” be used in locations defined as Exposure “A”.Use Exposure “C” for all buildings with heights 60 ft. or less. Remember to first correct the documented wind speed for the Importance Factor of the building based upon the intended use of the structure.
